CBCT Assessment of the Anatomical Characteristics of Gubernacular Canal in Impacted Teeth

Statement of the Problem: Gubernacular canal (GC) is a canal that extends from the follicle of unerupted permanent teeth to the alveolar bone crest filled with remnants of the dental lamina. This canal is thought to guide tooth eruption and be related to some pathologic conditions. Purposes: This study aimed to determine the presence of GC and its anatomical characteristics in teeth, which failed to erupt normally on cone beam computed tomography (CBCT) images. Materials and Method: This cross-sectional study evaluated CBCT images of 77 impacted permanent and supernumerary teeth obtained from 29 females and 21 males. The frequency of GC detection, its location in relation to the crown and root, the anatomical surface of the tooth from which the canal has originated, and the adjacent cortical table to which the canal opens, along with the length of the GC were studied. Results: GC was observed in 53.2% of teeth. The anatomical tooth aspect of origin was occlusal/ incisal in 41.5% and crown in 82.9% of teeth. Moreover, 51.2% of GCs opened in palatal/lingual cortex and 63.4% of canals were not located along the tooth long axis. Finally, GC was detected in 85.7% of teeth undergoing the crown formation stage. Conclusion: Although GC was introduced as an eruption pathway, this canal is also present in impacted teeth. This means that presence of this canal does not promise the normal eruption of tooth and the anatomical characteristics of GC may influence the eruption process.


Introduction
Tooth eruption is a physiological process involving a variety of anatomical components. Root development, alveolar bone growth, pulp development, and a mix of follicular theory and genetic factors, which is more widely acknowledged nowadays, are all explanations for this process [1][2]. The follicular theory postulates that dental follicles play an important role to guide the tooth and inducing bone resorption to create an eruption pathway during the intrabony phase [1][2][3]. Gubernacular canal (GC) and gubernacular cord were first introduced by John Hunter [4] . This canal originates from the folli-cle of permanent teeth and extends to the alveolar crest palatal/ lingual to deciduous teeth. It is filled with a gubernacular cord, which includes peripheral nerves, blood, and lymphatic vessels, as well as remnant epithelial cells of the dental lamina [2]. The gubernacular cord also contains many chemical mediators, including the epidermal growth factor (EGF), which has the ability to activate osteoclasts [2]. As a result, bone resorption occurs via this canal, whereby an eruptive pathway from the dental follicle to the gingiva is created [2,[5][6]. Although it was suggested that GC acts as a guide for tooth eruption, its presence was proved only in permanent successional teeth and those without deciduous predecessor, but its presence has not been definitely determined in primary teeth [5,[7][8]. However, it was stated that the absence of GC may indicate failure in tooth eruption [5], but this structure is observed in teeth with eruption failure too [9]. This makes the relationship between the failure of tooth eruption and GC characteristics unclear. Some studies [10][11] suggested that the gubernacular cord may play a role in the formation of adenomatoid odontogenic tumor (AOT) and ameloblastoma, since these tumors are composed of dental lamina remnants. The results of previous studies also have indicated a significant relationship between odontoma and the presence of GC [10][11].
On radiographic images, GC is described as a radiolucent/ hypodense canal with corticated boundaries connected to the dental follicle [7]. The delicate anatomic structure of GC, besides inherent superimpositions on two-dimensional radiographies makes its visualization on periapical and panoramic images difficult. GC can be mistaken for the nutrient canal, an accessory branch of the inferior alveolar canal, or even the fistular tract on radiographs regarding its position in the jaw. Nowadays, with the extensive use of cone beam computed tomography (CBCT) in dentistry, GC is detected more frequently by clinicians, and questions about its role in tooth development and eruption were raised. However, few clinical studies have focused on the presence, appearance, and importance of these structures. Hence, this study was designed to investigate the present rate of GC in impacted teeth (teeth that have failed to erupt at expected time) as well as anatomical characteristics of GC, which may be related to the eruption failure that occur in fully impacted teeth in alveolar bone by employing CBCT images in a population of south Iran.   The minimum, maximum and mean GC lengths were 1, 5 and 3.04 mm, respectively (SD= 3.04). The min- 1% in impacted teeth, teeth with delayed eruption, and teeth with normal eruption, respectively. Oda et al. [9] reported a detection rate of 90% in teeth with normal er- Nishida et al. [5] also reported that the GC detection rate in mesiodens teeth was far lower than in other teeth (1.6%). Although GC had previously been thought to be an eruption channel, new findings reveal that this canal is also seen in the impacted teeth, suggesting that the presence of this canal does not guarantee tooth eruption [5]. There was no significant association between GC prevalence and jaw and tooth type in this investigation.

Materials and Method
The current study revealed that the presence of GC is not limited only to the teeth with deciduous precursors; rather, there were also molar and supernumerary teeth with the GC. Consistent with the current study, Nishida et al. [5] and Ahmed et al. [6] reported the presence of GC in molar teeth. However, Hodson [8] believe that the GC is only present in teeth that erupt after the deciduous teeth were exfoliated. Gaêta-Araujo et al. [7] found that GCs extended more commonly in the occlusal/incisal direction in both normal and aberrant erupting teeth, however additional directions (mesial, distal, buccal, and palatal/lingual) were frequently detected in teeth with abnormal eruption. In the present study on impacted teeth, the extension of GC was seen more frequently in directions other than occlusal/incisal (58.5%).
It may result in GC in a developing tooth extending in occlusal/ incisal direction; it has more chance to erupt normally. It can be anticipated that if the GC in a developing tooth extend in occlusal/ incisal direction, the tooth has more chance to erupt normally. In the present study, which was performed on impacted teeth, the location of the GC was in the crown in 34 teeth. In the remaining 7 teeth with GC, its location was in the tooth cervix. Gaêta-Araujo et al. [7] classified the canal location in relation to the tooth structure as occlusal/incisal, middle of the crown, cervical, and root. The results of their study revealed that in 35 out of 144 teeth with abnormal eruption, the canal was located in the middle of the crown, cervix, and root. On the other hand, the GC had an occlusal/incisal location in 396 teeth with normal eruption. In a study on the GC characteristics in mesiodenses, Oda et al. [9] showed that the GC was located in the cervical and root regions of mesiodenses with inversely crown-root position, while it was mainly lo-cated in the crown of mesiodenses with a normal posi- [5] reported a mean canal length and width of 3 and 1 mm, respectively. Oda et al. [9] indicated in their study that the canal length and width were related to the eruption stage, and the closer the tooth was to the alveolar crest, the shorter the canal length would be. Gaêta-Araujo et al. [7] showed that the canal length was longer only in mandibular premolar teeth with delayed eruption and there was no significant difference between other groups in terms of their canal length. However, the canal width was significantly larger in impacted teeth and teeth with delayed eruption. In the present study, the impacted teeth were divided into four groups in terms of position status i.e. normal, angular (oblique), and inverted positions. Although the GC was seen in fewer teeth with normal position, this difference was not statistically significant. Gaêta-Araujo et al. [7] showed that there was a weak relationship between the rates of canal detection and the angle of tooth position in teeth with normal eruption. On the other hand, in impacted teeth, the detection rate of the GC was higher in horizontally positioned teeth than those with angular and normal positions (horizontal: 100%, angular: 86.2%, normal: 83.1%). Finally, based on this study's results, GC was found in 85.7% of the teeth in the crown development stage and 46% of those in the root formation stage. Gaêta-Araujo et al. [7] also concluded that the GC detection rate was significantly higher in the impacted teeth during the initial stages of tooth formation (100%).
This research has several drawbacks, including as a limited sample size and the absence of a control group of teeth with normal eruption. One suggestion is to examine imaging properties of GCs in children of various ages for better understanding the possible functions of GC in tooth growth and eruption.

Conclusion
GC can be detected in impacted permanent and supernumerary teeth with a lower rate than normal erupting teeth. In most of these teeth, GC is not along the tooth long axis. Additionally, extension of GC in impacted teeth was seen more frequently in directions other than occlusal/ incisal.